The objectives of the proposed research are: (1) To quantitate by means of an immunofluorescent technique the density and distribution of Gastrin G-cells in the dog antrum, (2) To seek correlations between G-cell density, antral mucosal gastrin concentration, and serum gastrin, fasting and food stimulated, (3) To investigate sequential alterations in G-cell populations, tissue and serum gastrin in experimental circumstances previously shown to cause hypergastrinemia and/or parietal cell hyperplasia. The latter is known to occur as a trophic response to elevated gastrin levels. Preparations to be studied involve antral distention, antral alkalinization, exclusion of bile and/or pancreatic juice from the duodenum, portacaval transposition, shortened small bowel, hypercalcemia, cortisone or growth hormone administration, vagal denervation and antrectomy. The overall goal is to define the pace of G cell mass as a determinant of gastrin production.